Grapevine, The International Journal of Alcoholics Anonymous, identified Niebuhr as the author (January 1950, pp. 6–7), and the AA web site continues to identify Niebuhr as the author.
The original text for this adapted prayer was:
Father, give us courage to change what must be altered,
serenity to accept what cannot be helped,
and the insight to know the one from the other.
https://en.wikipedia.org/wiki/Serenity_Prayer